Sharp rises in the cost of conventional fuels have made alternative sources of power, such as solar ponds, economically attractive. A solar pond is a body of standing water having a non-convective layer, called a halocline, located just below the surface of the water. The halocline, which is usually about 1-1.5 m deep, has a downwardly directed density gradient achieved by a salinity that changes from about 5% near the top of the halocline to around 30% near the bottom. Covering the halocline is a wind-mixed layer from 10-50 cm deep, depending on weather conditions, this layer having a uniform salinity of around 5%. Below the halocline is water of uniform salinity, about 30%, which forms the heat storage layer of the pond.
Solar radiation incident on the surface of the pond penetrates into the halocline and into the heat storage layer, and is absorbed, thereby heating the water. In the wind-mixed layer, heated water below the surface is lighter than the cooler surface water; and the differences in densities throughout the wind-mixed layer establish convection currents that rapidly transfer the warmer water in the wind-mixed layer to the surface, where the absorbed heat is dissipated to the atmosphere. The temperature of the wind-mixed layer thus approximates ambient temperature.
Water in the halocline heated by the absorption of solar radiation also becomes lighter; but the density profile of the halocline, which closely matches the salinity profile, ensures that the density of a lower heated stratum exceeds the density of the stratum immediately thereabove, with the result that convection currents in the halocline are suppressed. Consequently, after a period of time a temperature profile is established in the halocline which, in general, matches the salinity profile. Ultimately, the water in the heat storage layer is heated; and the heated water in this layer is protected against conductive heat loss to the atmosphere by the halocline, which thus acts as an insulator. In this manner, the temperature of the water in the heat storage layer can reach 90-100.degree. C.
Optimal performance of a solar pond requires deep penetration of solar radiation into the pond. If the water is turbid, less radiation penetrates into the heat storage layer, reducing the efficiency of the pond. Therefore, the clarity of water in a solar pond is of critical importance.
For large-scale solar ponds, i.e., those capable of producing electricity in the megawatt range, the bodies of water are so large that clarifying and maintaining the clarity of the water is a formidable problem. For example, a solar pond located at the latitude of southern California must have an area of about 1 km.sup.2 (10.sup.6 m.sup.2) in order to have the capability of supplying sufficient heat to generate about 3 MW of electricity on a continuous basis (i.e., 24 hours per day). Solar ponds of this size, whether converted from an existing body of water or created artificially, contain so much water that conventional techniques of water treatment to reduce turbidity are expensive and time-consuming.
Construction of a solar pond requires pre-clarification of the water that is to constitute the heat storage layer, i.e., clarification before the halocline is created above the heat storage layer. After the main body is clarified, the water with which the halocline is to be created must also be clarified.
The conventional approach to reducing turbidity in large standing bodies of water involves the construction of settling ponds, into which water from the main body is pumped and treated with flocculating material such as aluminum sulfate. To be effective, the flocculating material must be dispersed throughout the water being treated; and the usual procedure is to provide a mechanical mixing system.
Conventionally, a three-stage process is involved in treating the water with a flocculation agent: a rapid mixing stage in which the agent is mixed into the water to achieve uniform distribution; a slow mixing stage to distribute the turbidity-causing material with water to be trapped in the flocs; and a sedimentation stage in which the flocs settle to the bottom, thus clearing the water.
After the sediment has settled, the clarified water can then be returned to the main body of water. This pumping, mixing, and settling is cumbersome and, for large bodies of water, involves considerable time. After the main body of water is clarified, construction of the halocline can then take place; and only after the halocline is in place can the pond act as a solar collector. Conventional approaches to reducing the turbidity of large bodies of water therefore increase construction time and costs of a solar pond.
It is, therefore, an object of the present invention to provide a new and improved method of and apparatus for reducing the turbidity of a standing body of fluid wherein the problems encountered with prior art techniques are overcome or substantially reduced.